In various lighting applications (e.g., home, commercial, industrial, etc.), there are instances in which it is desirable to adjust the amount of light generated by one or more conventional light sources (e.g., incandescent light bulbs, fluorescent light fixtures, etc.). In many cases, this is accomplished via a user-operated device, commonly referred to as a “dimmer,” that adjusts the power delivered to the light source(s). Many types of conventional dimmers are known that allow a user to adjust the light output of one or more light sources via some type of user interface (e.g., by turning a knob, moving a slider, etc., often mounted on a wall in proximity to an area in which it is desirable to adjust the light level). The user interface of some dimmers also may be equipped with a switching/adjustment mechanism that allows one or more light sources to be switched off and on instantaneously, and also have their light output gradually varied when switched on.
Many lighting systems for general interior or exterior illumination often are powered by an A.C. source, commonly referred to as a “line voltage” (e.g., 120 Volts RMS at 60 Hz, 220 Volts RMS at 50 Hz). A conventional A.C. dimmer typically receives the A.C. line voltage as an input, and provides an A.C. signal output having one or more variable parameters that have the effect of adjusting the average voltage of the output signal (and hence the capability of the A.C. output signal to deliver power) in response to user operation of the dimmer. This dimmer output signal generally is applied, for example, to one or more light sources that are mounted in conventional sockets or fixtures coupled to the dimmer output (such sockets or fixtures sometimes are referred to as being on a “dimmer circuit”).
Conventional A.C. dimmers may be configured to control power delivered to one or more light sources in one of a few different ways. For example, in one implementation, the adjustment of the user interface causes the dimmer to increase or decrease a voltage amplitude of the A.C. dimmer output signal. More commonly, however, in other implementations, the adjustment of the user interface causes the dimmer to adjust the duty cycle of the A.C. dimmer output signal (e.g., by “chopping-out” portions of A.C. voltage cycles). This technique sometimes is referred to as “angle modulation” (based on the adjustable phase angle of the output signal). Perhaps the most commonly used dimmers of this type employ a triac that is selectively operated to adjust the duty cycle (i.e., modulate the phase angle) of the dimmer output signal by chopping-off rising portions of A.C. voltage half-cycles (i.e., after zero-crossings and before peaks). Other types of dimmers that adjust duty cycles may employ gate turn-off (GTO) thyristors that are selectively operated to chop-off falling portions of A.C. voltage half-cycles (i.e., after peaks and before zero-crossings).
FIG. 1 generally illustrates some conventional A.C. dimmer implementations. In particular, FIG. 1 shows an example of an A.C. voltage waveform 302 (e.g., representing a standard line voltage) that may provide power to one or more conventional light sources. FIG. 1 also shows a generalized A.C. dimmer 304 responsive to a user interface 305. In the first implementation discussed above, the dimmer 304 is configured to output the waveform 308, in which the amplitude 307 of the dimmer output signal may be adjusted via the user interface 305. In the second implementation discussed above, the dimmer 304 is configured to output the waveform 309, in which the duty cycle 306 of the waveform 309 may be adjusted via the user interface 305.
As discussed above, both of the foregoing techniques have the effect of adjusting the average voltage applied to the light source(s), which in turn adjusts the intensity of light generated by the source(s). Incandescent sources are particularly well-suited for this type of operation, as they produce light when there is current flowing through a filament in either direction; as the average voltage of an A.C. signal applied to the source(s) is adjusted (e.g., either by an adjustment of voltage amplitude or duty cycle), the current (and hence the power) delivered to the light source also is changed and the corresponding light output changes. With respect to the duty cycle technique, the filament of an incandescent source has thermal inertia and does not stop emitting light completely during short periods of voltage interruption. Accordingly, the generated light as perceived by the human eye does not appear to flicker when the voltage is “chopped,” but rather appears to gradually change.